As the track density employed on data-storage media continually increases, the need for accurate and precise head positioning becomes ever greater. Recording media having particularly high track densities may include, among others, hard or flexible disk memory and magnetic recording tape. Typically, to achieve precise head positioning, a servo head is coupled to the data head so that data head position is made a direct function of servo head position Accordingly, by controlling the position of the servo head, the data head can also be controlled, so as to remain properly aligned with a track upon which data is either being recorded ("written") or reproduced ("read").
In a typical servoing arrangement, the servo head is controlled by coordinating its position with servo information prerecorded on the media either along the data tracks or on one or more separate tracks. Currently, three types of servo systems are employed on magnetic media, namely amplitude-based, frequency-based, and phase or time-based. With the amplitude-based servo patterns, perceived differences in amplitude between successive sections of the servo pattern are used to obtain position information. Many such systems use various pulse-coding schemes as well, and a good example of an application of such an amplitude-based servo system relevant to the magnetic recording tape field is fully discussed, and incorporated by reference herein, U.S. Pat. No. 4,472,750, issued to Klumpp et al., which interleaves the servo fields with the data fields.
Pulse-type amplitude detection-based servo patterns have found widespread use in disk drives. These types of patterns are often found in a dedicated servo environment and are to some extent amplitude-based. Such servo patterns may employ so-called dibit, tribit, or quadbit formats to encode position information. Alternatively, pulse detection-based servo patterns may be embedded on disks or recording tape, wherein the servo information is added to a header portion of a data track or segment.
As well illustrated in prior U.S. Pat. No. 4,007,493, which shares common inventorship with the instant application, a time-based system may be implemented by using a pair of mutually-spaced servo tracks disposed along opposite sides of a data track. This system operates by comparing the timing differences resulting from detection of the transitions in the two tracks, generated by moving the storage media (i.e., rotating a disk) under two spaced heads. More specifically, the two such servo tracks have different servo line densities and when moved under the associated heads thus generate output pulse trains of different pulse repetition rates. The positioning of sensing transducers in such a system is accomplished by measuring the time relationship between the output pulses from the two different servo tracks. While such a servoing scheme represents a potentially great advance over amplitude-based servoing, this particular system still possesses some of the disadvantages typically associated with prior art dedicated servo systems. More specifically, as disclosed, this prior patent teaches the use of two substantially different servo tracks, and such tracks are formed by recorded servo lines which use the same surface as the data tracks, with the data lines adjacent to but segregated from the servo lines. Consequently, a considerable portion of the disk in such a system is dedicated to tracks of servo patterns, upon which data cannot be recorded. Additionally, the use of two separate and different mutually-spaced servo tracks to position the transducer for a single data zone creates complex problems in actual implementation and further diminishes the amount of space on the record member which can be devoted to data.
In order to free up additional space for data tracks on recording media, and thus improve the efficiency of data storage on the media, "burying" the servo patterns beneath the surface upon which data is written has been proposed heretofore. For example, buried servo schemes have been disclosed in such references as U.S. Pat. No. 3,614,756 (McIntosh), U.S. Pat. No. 3,956,769 (Beecroft et al.), and U.S. Pat. No. 4,581,663 (Tanaka). Many problems have arisen with the employment of buried servo pattern arrangements as disclosed in the prior art, however, particularly since in both amplitude-based and frequency-based systems the same head is used to read both user data and servo information. Thus, serious problems arise in distinguishing one such type of information from the other, and amplitude-based servo systems experience other serious problems due to amplitude reduction accompanying increased track density, i.e., signal amplitude is lost as the track width decreases. Furthermore, amplitude detection errors naturally result from the employment of a servo layer which is "buried" directly beneath the user data. Frequency-based systems experience great difficulty in separating two different servo frequencies from one another and from the data signal frequency. While the Tanaka system appears to be time-based, its intended use is for a single data track. Hence, the use of the Tanaka system to function as a multiple-track positioning device is neither actually proposed nor clear once conceived.